This invention relates generally to seismic exploration of substrata beneath bodies of water and, more particularly, to marine seismic exploration conducted by sensing converted shear waves reflected from such substrata in response to a downwardly travelling compressional wave.
Marine seismic exploration is generally conducted by towing a seismic streamer at a given depth through the ocean or other body of water. The streamer is provided with a plurality of pressure sensors, such as hydrophones, disposed at appropriate intervals along its length. Compressional wave energy is provided in the vicinity of the cable by an air gun or other suitable means; this compressional wave energy travels downwardly through the earth with a portion of it being reflected upwardly at levels where there is a contrast in the acoustic impedance characteristics of the strata. The pressure sensors detect the compressional waves produced in the water by the upwardly travelling seismic reflections and provide electrical signals indicative thereof to suitable processing and recording equipment located on the seismic vessel that is towing the streamer. It has been found that shear waves are generated from the compressional waves at interfaces in the strata; these shear waves contain additional information on the nature of the strata. However, this data is not considered since the reflected shear waves are not sensed by the marine seismic systems of the prior art.
Therefore, it is an object of the present invention to provide a geophone assembly suitable for use with a marine cable to detect the converted shear waves reflected from the strata during seismic exploration with a compressional source.